1. Field of the Invention
This invention generally relates to nuclear meters, and particularly to an apparatus or a method for absorbing and otherwise controlling unwanted escaping neutrons from a neutron-capture-based elemental analyzer for on-line measurement of bulk substances.
2. Description of the Prior Art
The rising cost of fuels, coupled with the need to avoid atmospheric pollution when burning them, has led to the requirement that their composition be known at various points in the fuel-preparation cycle. For example, because of the scarcity of low-sulfur crude oils and the cost of sulfur removal, the value of fuel oil increases significantly as its sulfur content becomes lower, indicating that accurate fuel-oil blending to a fixed sulfur level consistent with allowable amounts of pollution is both cost effective and an efficient utilization of increasingly-scarce hydrocarbons. Furthermore, precise knowledge of the heat content of fuel oil allows furnaces and boilers to be operated in a more efficient manner. In addition, knowledge of the amount of sulfur and other contaminants such as vanadium and nickel in various hydrocarbon streams can help prevent the poisoning of catalysts used in oil refineries, avoiding costly shut downs.
In the case of coal, sulfur content is generally higher than that of oil, making the pollution problem even more severe. As a result, expensive coal-cleaning plants, stack-gas scrubbers and precipitators are necessary, all of which can be operated more efficiently if the coal consumption is known on a real-time, on-line basis. Efficient boiler operation also benefits from this composition measurement, and knowing the composition of the ash in the coal can be used to avoid boiler slagging, which is a costly problem that is generally absent for fuel oil.
Particularly in the case of coal, but also for oil, these composition measurements have to be made on inhomogeneous substances with high mass flow rates and variable compositions. Thus, this measurement should continuously reflect the average composition of the bulk substance, and response times should be fast enough to permit effective process control. Generally the latter requirement implies a speed of response ranging from a few minutes up to an hour.
A technique which can satisfy these requirements can often be used in applications which do not involve fuels or their derivatives. For example, it could measure the nitrogen content of wheat in order to determine the amount of protein present, which in turn is related to food value. Thus, the measurement of fuels is illustrative only and is not essential to this invention, which applies to all measurements of bulk substances by the techniques to be described hereinafter.
Several methods for composition measurement are known in the prior art, the most obvious one being sampling followed by chemical analysis. This technique provides most present data on the composition of various bulk substances. Unfortunately sampling is inherently inaccurate because of the lack of homogeneity of bulk materials, and large continual expenditures for manpower, sampling devices and chemical-analysis equipment are required to provide response times which at best could approach one hour. These disadvantages lead to the consideration of other techniques which are faster, more subject to automatic operations and more of an on-line continuous bulk measurement.
One technique often used in industrial environments for elemental analysis involves X-ray fluorescence. This technique relies on the fact that each atom emits X rays with distinct and well-known energies when external radiations disturb its orbital electrons. Unfortunately, sulfur, which is an interesting element from the standpoints of air pollution and catalyst poisoning, emits mostly 2-keV X-rays, which can only traverse about 0.1 mm of a typical fuel. Iron, which is one of the elements generating the highest-energy X rays in coal, produces mostly a 6-keV X ray, which also cannot escape from any appreciable amount of coal or other nongaseous fuel. Thus, the use of X-ray fluorescence for other than gaseous materials requires either the preparation or the vaporization of a sample in an atmosphere which does not confuse the measurement. In either case, a difficult sampling and sample-preparation problem compounds the errors associated with X-ray fluorescence itself.
A second technique usually involving X rays which are more penetrating is X-ray absorption. In this case one measures the differences in the absorption or scattering of X rays caused by changes in the amounts of certain elements. In the case of relatively-pure hydrocarbons such as refined fuel oil, this technique can provide a useful measurement of sulfur content because sulfur at X-ray energies near 22 keV can have a predominant effect on the X-ray absorption. This predominance, however, is dependent on the lack of most of the metals which are present in coal and may also be present in oil. In addition, 22-keV X rays only penetrate about 2 mm in most non-gaseous fuels, making sampling still a requirement. Moreover, this technique is generally limited to measuring only one of several potentially-interesting elements, and the measurement of the relative amounts of many different elements in a complex mixture such as coal becomes difficult.
Nonetheless, nuclear techniques in general remain attractive because they often can be automated and in principal do not require actual manipulation of the bulk material itself. The problems with X-ray fluorescence and absorption arise partly because the associated radiations are not sufficiently penetrating. However, because the energetic gamma rays produced by the capture of thermal neutrons will penetrate over 100 mm of most fuels, an analysis technique based on them can provide an accurate, continuous, on-line measurement of the elemental composition of bulk substances without sampling.
This technique is based on the fact that almost all elements when bombarded by slow neutrons capture these neutrons at least momentarily and form a compound nucleus in an excited state. Usually the prompt emission of one or more gamma rays with energies and intensities which are uniquely characteristic of the capturing nucleus dissipates most of this excitation energy. Because these prompt gamma rays often have energies in the 2- to 11-MeV range, they can penetrate substantial quantities of material. Thus, for those isotopes with significant capture cross sections and prominent gamma-ray lines, measurement of prompt gamma rays can be used to determine in an on-line, real-time basis the quantity of most of the elements present in bulk substances, which can be flowing through the analyzer.
The above emphasis on thermal neutrons reflects the fact that for most elements the cross section for neutron capture is approximately proportional to the reciprocal of the square root of the neutron energy. Thus, almost all neutron capture occurs at the lowest neutron energies, which happen when the neutrons are in thermal equilibrium with the nuclei of the surrounding medium. As a result, the thermal-neutron-capture cross sections characterize the expected prompt-gamma-ray spectra. These gamma-ray spectra are particularly amenable to simple theoretical interpretation using well-known thermal-neutron-capture cross sections, making automatic operation a feasible concept.
However, because isotopic and other neutron sources generally produce neutrons with average energies of at least several MeV, "moderation" or "thermalization" processes must reduce neutron energies by over eight orders of magnitude in order for them to reach the thermal region near 0.025 eV. Collisions with hydrogen nuclei, which have a mass essentially the same as that of the neutron and a large scattering cross section, are the most effective means for neutron moderation, although collisions with other elements will moderate neutrons to some degree. Because the neutrons move between collisions, the volume of material exposed to significant neutron fluxes can have a considerable extent, which depends mostly on the amount of hydrogen present. Because the thermal neutrons are produced continuously by moderation of the more-energetic neutrons and then diffuse throughout this moderation volume, the substance being measured is sampled over a large extent, providing the bulk measurement.
Although these techniques have been used in the laboratory under controlled conditions, their implementation in an automatic, on-line instrument placed in an industrial environment presents unique problems. One of these problems results from the extended neutron cloud, which provides the bulk measurement. Because the sources of prompt gamma rays follow the flux of thermal neutrons, an extended neutron cloud implies that the gamma rays may have to travel a substantial distance in order to leave the measurement volume and enter an external detector where their energy can be measured. As their path length in the material increases, the probability that they will scatter also increases, decreasing the number of useful events while increasing the unwanted background in the measured gamma-ray spectrum. As a result, the "signal-to-noise ratio" of the measured spectrum decreases, adding to errors and difficulties in automatic data analysis, as well as increasing statistical variations causing larger fluctuations in the measured elemental composition and longer response times. For industrial applications such drawbacks are sufficiently severe that this technique has not been generally used heretofore.
In the prior art, the average gamma-ray path length was longer than that resulting purely from the desire to make a bulk measurement. This added path length resulted from the need to absorb essentially all of the neutrons in the measurement volume in order to prevent escaping neutrons from producing an intolerable background. This background resulted from neutron interactions in the gamma-ray detector itself and from gamma rays produced by neutron capture in materials surrounding the measurement volume.
These difficulties in the prior-art instruments arose partly because materials absorbing escaping neutrons without generating interfering gamma rays were not placed around major portions of the measurement volume. Often iron-containing metals surrounded the measurement volume, and because iron captures neutrons readily, the resulting gamma rays added to the background and to the difficulties in measuring iron in the bulk substance being analyzed. At best those instruments attempted to absorb thermal neutrons entering the gamma-ray detector directly by placing a thin layer of boron around the detector. Unfortunately boron produces a low-energy gamma ray after neutron capture, and this gamma ray added to the background counting rate. As a result, those instruments often had lead mixed with the boron in order to absorb or scatter low-energy gamma rays, but as a result the scattering of the interesting energetic capture gamma rays was also increased, decreasing the signal-to-noise ratio in the measured gamma-ray spectrum. In addition, the boron layer was too thin to absorb any appreciable fraction of the energetic and epithermal neutrons, which represent the majority of the escaping neutrons. Because iodine used in common gamma-ray detectors such as NaI(Tl) or CsI(Na) reacts readily with epithermal neutrons by resonance absorption, the thin boron layer did not remove a significant contributor to the neutron-induced background. Thus, in these prior-art instruments the measurement volume had to be large enough to prevent significant numbers of neutrons from escaping. Because this size was greater than that needed for handling the bulk substance, added gamma-ray scattering reduced instrument accuracy. Additionally a potential radiation hazard to personnel in the vicinity of the instrument existed whenever the measurement volume was empty with the neutron source present.